How can I interest people in topics that they are uninterested in?

My uncle insists I could sell sand to bedouins. My mother and father drool when I speak. My little cousins and some friends find me fascinating. My sister and other friends openly tell me to shut up.
The difference? How I communicate with each group.
As a general rule, if there’s rapport, and you project certainty and passion, something will get to the other side. If you don’t have rapport, build it. Care for what the other person thinks and feels. Once they realize you are genuinely interested in them, they’ll be more open to what you’re interested in.
A caveat: people usually have topics they have aversion towards. First thing that comes to mind is this friend who, since he feels inferior, blindly holds this faccade of being perfect and awesome at everything all the time. Anytime something fails, it’s not his fault. So everytime I mention any topic which could threaten that false sense of security. He shuts off. It comes across as not interested, but it’s different.
That being said, remember that we are mainly emotional animals with a paintjob of logic on top. Not the other way round. So if you go all facts, you’re done for. Unless it’s a science profile who feels really strongly about logic and science and facts. Go for the Why. Paint a word scene. Tell an emotionally impacting story.
Basically the opposite of what I just did. :P It’s not they way most of us communicate. Takes a little effort, but after a few tries, it gets really fun.
Talk as is what you’re saying is the most interesting subject in the world four you right now. If it isn’t, if something else is, you should be talking about that instead.
If they care about you, some of your interest will rub off on them. If they don’t, work on that first. Listen. Care.